Bollettino valanghe

Northwest Mountains

Crested Butte Avalanche Center
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Pericolo valanghe

Sat
Alta quota
4 · Forte
Limite del bosco
4 · Forte
Sotto il limite
3 · Marcato
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Non aggiornato — questo bollettino è scaduto 6 months ago. Stai vedendo condizioni passate, non il bollettino in corso.

In sintesi

Very dangerous avalanche conditions prevail as this week’s heavy snowfall has pushed the weak snowpack near its breaking point. The potential for a serious or deadly accident is very real. Large and destructive avalanches may occur without obvious warning signs. Another winter storm today will maintain the possibility for natural avalanches, which could run far into lower-angle terrain and lower elevations. Southerly-facing slopes offer less avalanche risk than northerly-facing slopes. Use the avalanche terrain map to plan lower-angle routes that avoid avalanche start zones and runouts.

 

Problemi valanghivi

  • Persistent Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    2–3

    Persistent slab avalanches will break an average of 2 to 4 feet deep on buried persistent weak layers, and may run naturally today during periods of active loading. These slabs have settled and are not often showing obvious warning signs before large and destructive avalanches release. The north half of the compass has the most fragile weak layers buried in the upper and lower snowpack (see videos 1 & 2) and is most likely to produce natural releases, remote triggers from flat terrain, widely propagating, and destructive avalanches.  Southerly aspects have a more springlike snowpack structure, reducing the likelihood that avalanches break deeper and larger than the storm snow.  Manage today's persistent slab problem by avoiding travel on or below terrain steeper than 30 degrees on problematic aspects, including runout zones.

    A waist-high slab that was remotely triggered in Evan's Basin on Wednesday. The size of these avalanches will grow even further with higher elevations and/or more wind-loading. 

  • Wind Slabs

    • Alta quota
    • Limite del bosco
    • Sotto il limite del bosco
    Probabilità
    Likely
    Dimensione
    1.5–2.5

    The average wind direction since Wednesday has been from the southwest to the west. There has been ample snow available for transport. This morning, winds will increase from the south before backing to the west. Thick wind slabs have been created on the lee sides of ridgelines and cross-loaded terrain features. Wind-loaded terrain is most common at near and above tree line elevations, while a few lower elevation slopes have also been wind-loaded. The simple travel advice is to avoid wind-loaded terrain features on the lee sides of ridgelines and cross-loaded terrain features. 

    A thick cornice that was remotly triggered on Wedneday in Evan's Basin. 

Avalanche Discussion

Decent views into the mountains yesterday revealed sizeable natural avalanches from Wednesday's natural cycle, and didn't reveal any notable fresh natural avalanches that ran yesterday. One of the most notable observations is the number of slopes that remain intact and are holding most, if not all, of the snow that has accumulated this week. The size of those slabs have deadly and destructive potential. Let that point sink in. This is not an avalanche problem that you can ski cut and manage, and if you get caught, the situation wouldn't be good. 

The snow that has accumulated this week has settled, and the persistent slabs are thick enough that the weight of a human is not easily affecting buried weak layers. While this does decrease the chance of human triggering, it creates real accident potential, as obvious warning signs do not always precede avalanche release. This point was illustrated in this observation yesterday. Forcasters traveled above avalanche terrain for a couple of hours without any feedback from the avalanche problem, before suddenly traveling through a trigger point and remotely triggering an unsurvivable avalanche. 

Wind loading alone wasn't able to hold HIGH danger across the forecast area yesterday. The avalanche warning has expired in the Southeast Mountain forecast area. Don't be misled: this forecast area is still under dangerous avalanche conditions, with the possibility of natural avalanche activity today. HIGH avalanche danger and an avalanche warning continue for the NW Mountain Forecast area. Close to an inch of water weight could accumulate in the Northwest Mountain forecast area for this 24-hour period, combined with additional wind-loading; this could be enough to tip the scales back to HIGH avalanche danger. 

SWE totals since Tuesday are about 4.1" at Lake Irwin, 3.8" at Schofield,  1.8" at Upper Taylor, 1.4" at Butte. These remote weather stations were affected by the strong to extreme winds on Tuesday, and in some cases may be over-recording what actually fell from the sky, but they still help give a ballpark. Two weather stations reported total snowpack height changes of around 25" at 11,000ft on Purple Ridge and 23" at 11,000ft on Mt Emmons. Total HST measurements are no longer accurate at remote weather stations, as snowpack settlement has sometimes exceeded accumulating snow.  

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